ST_ToSurface method for type ST_Geometry

ST_Surface
If the geometry-expression is of type ST_Surface, return the geometry-expression.
If the geometry-expression is a geometry collection with a single element of type ST_Surface, return that element.
If the geometry-expression is the empty set, return an empty set of type ST_Polygon.
Otherwise, raise an exception condition.

The spatial reference system identifier of the result is the same as the spatial reference system of the geometry-expression.